All matter is made from atoms.
Every substance (oxygen, lead, silver, neon ...) has a unique number of
protons, neutrons, and electrons.
Oxygen, for example, has 8 protons, 8 neutrons, and 8 electrons.
Hydrogen has 1 proton and 1 electron.
Individual atoms can
combine with other atoms to form molecules.
Water molecules contain two atoms of hydrogen H and one atom of oxygen O
and is chemically called H2O.
Oxygen and
nitrogen are the major components of
air
and occur in nature as
diatomic (two atom) molecules.
Regardless of the type of molecule, matter normally
exists as either a solid, a liquid, or a gas.
We call this property of matter the phase of the matter.
The three normal phases of matter have unique characteristics which are listed on the
slide.

Solid

In the solid phase the molecules are closely bound to one another
by molecular forces.
A solid holds its shape and the
volume
of a solid is fixed by the shape of the solid.

Liquid

In the liquid phase the molecular forces are weaker than in a solid. A liquid
will take the shape of its container with a free surface in a gravitational
field. In microgravity, a liquid forms a ball inside a free surface. Regardless
of gravity, a liquid has a fixed volume.

Gas

In the gas phase the molecular
forces are very weak. A gas fills its container, taking both
the shape and the volume of the container.

Fluids (Liquids and Gases)

Liquids and gases are called fluids because
they can be made to flow, or move.
In any fluid, the molecules themselves are in constant, random motion, colliding with
each other and with the walls of any container.
The motion of fluids and the reaction to external forces are described
by the
Navier-Stokes Equations,
which express a conservation of
mass,
momentum, and
energy.
The motion of solids and the reaction to external forces are described by
Newton's Laws of Motion.

Any substance can occur in any phase. Under
standard atmospheric conditions,
water exists as a liquid. But if we lower the
temperature
below 0 degrees Celsius, or 32 degrees Fahrenheit, water changes its
phase into a solid called ice.
Similarly, if we
heat
a volume of water above 100 degrees Celsius, or 212 degrees Fahrenheit,
water changes its phase into a gas called water vapor.
Changes in the phase of matter are physical changes, not
chemical changes. A molecule of water vapor has the same chemical
composition, H2O, as a molecule of liquid water or a molecule
of ice.

When studying
gases ,
we can investigate the motions and interactions
of individual molecules, or we can investigate the large scale action
of the gas as a whole. Scientists refer to the large scale motion of
the gas as the macro scale and the individual molecular
motions as the micro scale. Some phenomenon are easier to
understand and explain based on the macro scale, while other
phenomenon are more easily explained on the micro scale. Macro scale
investigations are based on things that we can easily observe
and measure. But micro scale investigations are based on rather
simple theories because we cannot actually
observe an individual gas molecule in motion. Macro scale and micro
scale investigations are just two views of the same thing.

Plasma - the "fourth phase"

The three normal phases of matter listed on the slide have been known
for many years and studied in physics and chemistry classes.
In recent times, we have begun to
study matter at the very high temperatures and pressures which typically
occur on the Sun, or during re-entry from space. Under these conditions,
the atoms themselves begin to break down; electrons are stripped from
their orbit around the nucleus leaving a positively charged ion
behind. The resulting mixture of neutral atoms, free electrons, and charged
ions is called a plasma. A plasma has some unique qualities that
causes scientists to label it a "fourth phase" of matter. A plasma is
a fluid, like a liquid or gas, but because of the charged particles present
in a plasma, it responds to and generates electro-magnetic forces. There
are fluid dynamic equations, called the Boltzman equations, which include
the electro-magnetic forces with the normal fluid forces of the Navier-Stokes
equations. NASA is currently doing research into the use of plasmas
for an ion propulsion system.